Fix nightly release process

Previously, create-ko-yaml didn't need to be run in ${PROJECT_ROOT}, but
after tektoncd#4260 it tries to `go run` something assuming it's in
${PROJECT_ROOT}, which fails unless we `cd` into that directory.

tekton/publish.yaml

@@ -66,6 +66,8 @@ spec:
       #!/bin/sh
       set -ex
 
+      cd ${PROJECT_ROOT}
+
       # Combine Distroless with a Windows base image, used for the entrypoint image.
       COMBINED_BASE_IMAGE=$(go run ./vendor/github.com/tektoncd/plumbing/cmd/combine/main.go \
         gcr.io/distroless/base:debug-nonroot \